1. Field of the Invention
The present invention relates to a tape measure with an automatic blade extension mechanism.
2. Description of the Related Art
Generally, a tape measure includes a bobbin which is elastically supported in a housing. A blade is provided in the housing while being wound around the bobbin in a coil form. An end hook coupled to an end of the blade is exposed to the outside of a slot that is formed at a lower position on a surface of the housing. Thereby, as the end hook is pulled out, the blade is extended from the housing. Meanwhile, when the end hook is released, the blade is retracted into the housing due to the elasticity of a spring.
Further, a certain type of tape measure is provided with a locking means. The locking means functions to prevent the blade from being retracted into the housing due to the tension of the spring, even if the blade is pulled out of the housing and a pulling force applied to the end hook provided at the end of the blade is discontinued.
A tape measure having such a blade locking means is disclosed in Korean Patent Laid-Open Publication No. 2003-0065290, which was filed by the applicant of this invention and is titled “tape locking device for tape measures”. The document is incorporated into the present invention.
According to this document, the tape locking device includes a locking lever which is hinged to an upper end of a housing. A coupling piece extends downwards from the locking lever, and a cylinder wound with a coil spring is coupled to an end of the coupling piece. Further, a wedged stopper is mounted to an end of the cylinder to impart a high frictional force. The stopper is elastically inserted between an outer circumference of a tape wound around a bobbin and a linear tape pulled out through a slot, so that the extended tape is automatically stopped, due to friction between the surface of the stopper and the surface of the extended tape.
Meanwhile, a conventional tape measure is configured so that a user measures the dimension of a desired object using both hands. Thus, when it is impossible to use both hands, for example, when the user is holding a tool in one hand, the user cannot measure the object using the tape measure. In order to solve the problem, a tape measure capable of being used to measure a dimension of an object using one hand has been proposed in U.S. Pat. No. 5,875,987, which is titled ‘power operated tape measure’. According to the patent, the tape measure includes a casing having a battery chamber and a circular chamber. A motor is installed in the casing and is provided with an output shaft. A first switch is mounted on the casing and is electrically connected to the battery chamber of the casing to control the motor. An annular reel is fitted within the circular chamber. Further, a spring-biased tape is wound around the reel. A driving wheel is fixedly mounted on the output shaft of the motor to directly engage with the surface of the tape and thereby push the tape out of the casing. A rod is connected to the driving wheel. Further, a second switch is mounted on the casing and connected to the rod. The second switch disengages the driving wheel from the tape surface so as to allow the tape to be taken up.
The term “tape” used in the patent refers to the same part as the term “blade”. Generally, the term “tape” is used interchangeably with the term “blade”.
Meanwhile, the conventional tape measure is problematic in that the motor and other components are installed in the tape measure, so that the tape measure is heavy, thus it is inconvenient to use. Further, since the components are complex, manufacturing costs of the tape measure are high.
Moreover, since the motor is operated using the battery, the tape may sag due to the weight of the tape, when the tape is pulled out from the casing about 6 feet or more. In this case, the tape comes into contact with a lower edge of the slot, thus generating frictional force. Due to the frictional force, the tape extension speed is considerably reduced, and the motor has difficulty in extending the tape from the casing.
Further, when the tape is lengthily extended from the casing, the tape comes into contact with the lower edge of the slot, thus generating the frictional force. Due to the frictional force, the tape is not easily retracted into the casing when taken up.